Late last year, print readers of The New York Timesdiscovered a full-page color ad, signed by a group called “Appreciative New Yorkers,” touting former mayor Michael Bloomberg’s policy achievements. “Thank you, Mayor Bloomberg and your administration, for all that you have done for New York City,” it read in large lettering. Who are these thankful New Yorkers with a spare $70,000 to spend on praising a politician? Not even the Times knows.

Usually the Times will disclose the names of advertisers when asked, even if the ads themselves are unclear—for example, when a PR firm owned by the personal flack of Roger Ailes purchased two Times Book Review ads for Zev Chafets’ 2013 Ailes biography, in an effort to distract from Gabriel Sherman’s unauthorized biography of the Fox News chief.

Whoever wanted to thank Mayor Bloomberg, however, took unusual steps to make sure they couldn’t be identified. According to a Times executive briefed on the matter, the business side agreed to an arrangement in which the buyer needed only to submit an alias and a billing address, which the buyer supplied only after Times personnel promised not to divulge it. Payment was made with an untraceable traveler’s check.

The business side never determined the buyer’s identity. “The entire process was extremely secretive,” the executive said.

Citing the secrecy agreement, the executive would not reveal the billing address or even its general location. The executive did disclose that the address was not on East 79th Street or Lexington Avenue—the locations, respectively, of Michael Bloomberg’s Upper East Side townhouse and the glassy headquarters of Bloomberg LP.

Searches of various databases for businesses and charities did not turn up any groups called Appreciative New Yorkers. The only clearly identifying information contained in the ad itself was an email address, appreciativenewyorkers@gmail.com. A request for comment sent to that address received no reply.

According to Google’s password-recovery process, the account owner signed up under the name of “Henry Hudson,” the English sea explorer after whom the Hudson River was named. Less identifying, but perhaps more telling, is the ad’s weird kerning—in dozens of places, random letters overlap—which suggests the ad was created by a layman instead of, say, someone at Bloomberg Businessweek’s renowned art department.